Acoustic attenuator and method of attenuation of noise

ABSTRACT

A sound reducer or sound attenuator for use in cabinets and the like with the sound reducer comprising a plurality of small bead like acoustic resonators loosely held in position between two members with each of the acoustic resonators having an acoustical resonance cavity therein for generating a standing wave with the standing wave producing vibration displacement of the acoustic resonators thereby dissipating acoustical sound energy through mechanical work.

CROSS-REFERENCE TO RELATED APPLICATIONS, IF ANY

[0001] Not applicable

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

REFERENCE TO A MICROFICHE APPENDIX, IF ANY

[0003] Not applicable.

FIELD OF THE INVENTION

[0004] This invention relates generally to sound reduction and more particularly to reducing the internal noise from within a cabinet or the like by conversion of acoustical energy into mechanical or vibration energy.

BACKGROUND OF THE INVENTION

[0005] One of the difficulties with equipment and particularly with equipment confined in equipment cabinets is that the equipment generally generates noise within the cabinet which is often enhanced by vibration of the cabinet. In order to reduce the internal cabinet noise it is necessary to provide some type of acoustic damping. One of the ways of reducing unwanted sounds is to place materials that absorb the sound energy proximate the noise source. The present invention provides an improved method and apparatus for reducing internal noise within a cabinet by converting the acoustic energy of the noise into mechanical or vibration energy and then dissipating the mechanical or vibration energy through a plurality of separate acoustic resonators that are displaceable with respect to each other. The separate acoustical resonators can be placed in the cabinet or proximate the walls of the equipment cabinet to enable various acoustical frequency to be converted into mechanical energy and then dissipated as the energy is absorbed by vibrationly displacing the acoustical resonators.

SUMMARY OF THE INVENTION

[0006] Briefly, the present invention is an acoustic attenuator for use in cabinets and the like with the acoustic attenuator dissipating sound through conversion of acoustic energy from one state to another state with the acoustic attenuator comprising multiple small bead-like acoustic resonators held in a loosely packed condition to enable vibration displacement of the acoustic resonators with respect to one another. Each of the acoustic resonators having an acoustic resonance cavity therein for sound waves to enter therein. The resonance cavity therein, which functions as a Helmholtz resonator, generates a standing resonance wave with the standing wave resulting in vibration displacement of the non-secured acoustic resonators thereby frictionally and mechanically dissipating the sound energy before it can emerge from the cabinet.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007]FIG. 1 is a perspective view of a cabinet having a portion of a side of the cabinet cutaway to reveal an acoustic liner therein;

[0008]FIG. 2 is a partial sectional view of an acoustic liner for attenuating the sound level from one side to the other side of the acoustic liner;

[0009]FIG. 3 is an enlarged view of an acoustic resonator located in the acoustic liner of FIG. 2; and

[0010]FIG. 4 is a cross sectional view of the acoustic resonator of FIG. 3 taken along liners 44 of FIG. 3.

DESCRIPTION OF THE INVENTION

[0011]FIG. 1 shows a perspective view of an equipment cabinet 10 having a front panel 11 and a side panel 12 which has been partially cutaway to reveal a hollowed out region 16 between top member 15 and bottom member 14. Located in the hollowed out region 16 is a plurality of bead like acoustic resonators 13. In the embodiment shown multiple acoustic resonators 13 are located in an integral pocket of the cabinet 10 with the cabinet holding equipment such as a motor or the like (not shown) that normally generates annoying if not harmful noise levels.

[0012] The present invention is well suited for applications such as ocean going ships where equipment is concentrated in certain areas of the ship and the overall noise level increases due to the noise from multiple sources. The use of a noise reduction cabinet of the present invention can be achieved without increased use of space as the noise attenuation can be achieved within the cabinets. The result is the equipment operators and crew is less fatigued while space availability remains the same.

[0013]FIG. 2 shows a partial cross-sectional view of a liner 20 or sound attenuator for attenuation of the sound from one side to the other side of liner 20. The liner 20 comprises a base 24 on one end and a top member 24 a having sound penetrateable member such as a mesh liner 22 located on one side thereof. Located on the opposite side of base 24 and top is a second member 21 which could be an outside panel or the like that forms a region 25 there between. Second member 21 could include end caps (not shown), which would cooperate with the base and top member to form a complete enclosure or cavity 25. A plurality of acoustic resonators 23 is located in a loosely packed condition in cavity 25 which is located between members 22 and 21. The plurality of acoustic resonators 23 comprise bead like members of different size and shape and each having at least one acoustical resonant cavity therein. The acoustic resonators 23 are stacked between members 21 and 22 so as to have the outside surfaces in contact with each other and generally have a shape so that the acoustic resonators provide at least some air space around each other if the acoustic resonators are randomly positioned therein. The acoustic resonators are held in position proximate each other through gravitational forces with spaces or gaps between the outer surfaces of the acoustic resonators thereby allowing the acoustic resonators to be displaced or move in response to generation of an acoustic wave within the acoustic resonator 23. In addition to the acoustic resonators 23 the positioning of the acoustic resonators 23 within the cabinet provides a torturous path for sound waves to pass therethrough thereby further attenuating the sound waves by having the sound waves propagate through a labyrinth of passages formed between the exterior surfaces of the acoustic resonators.

[0014] The liner 20 can be placed on the interior of the cabinet 10 or preferably proximate the interior walls of cabinet to attenuate the sound before it escapes from the cabinet. The liner 20 can be formed as an integral part of the cabinet wall by placement in the hollowed out regions of the cabinet walls. The liner 20 can also be placed within the cabinet since sound intercepted by the liner 20 will be attenuated by the acoustic resonators located within the liner 20.

[0015] Referring to FIG. 3 there is shown a front view of a bead-like acoustic resonator 30 having an exterior surface 32 and a circular opening 31 therein. Each of the acoustic resonators 30 has an acoustical resonance cavity that functions as a Helmholtz resonator in that the acoustical energy is funneled into a single resonate frequency. In the present invention the acoustical energy is then transformed into vibration or mechanical energy to displace the resonators 30. In order to reveal the interior of acoustic resonator 30 a section line 4-4 has been cut through acoustic resonator 30 to reveal a cylindrical acoustical resonance cavity 36 having a length L located therein. For illustration purpose, located within cavity 36 is a standing sound wave 35 that corresponds to a resonant length of the acoustical resonance cavity 36. The generation of a standing wave within cavity 36 transfers vibration energy to the attenuator 30 which has a mass m₁. The result is that the acoustic energy causes bead 30 to vibrate back and forth within the plurality of acoustic resonators since the beads are maintained in a non-fixed condition. That is, the beads 30 with the acoustical resonant cavities therein are separate individual entities that can be displaced relative to one another. If the beads 30 are made of materials such as ceramic or glass the energy is dissipated fairly quickly. Other materials could be used for acoustical resonator beads since the material of the bead only partially determines how much energy is dissipated. Thus the acoustical energy on one side of liner 20 is converted into vibration energy thereby attenuating the sound waves leaving member 21 and as a result reducing the noise level to those persons located proximate the equipment cabinet. A mock up of the system using the plurality of bead-like acoustic resonators resulted in a reduction of 6 db with the use of the aforedescribed acoustic liner. It should be appreciated that more or less acoustic damping can be obtained by using more or less acoustic resonators. In addition, by changing the length or shape of the cavities one can produce acoustic resonators that dissipate sounds over a range of frequencies. Thus to accomplish reduction of a particularly frequency range of sound waves one can tune the liner by providing the acoustic resonators with acoustic resonate cavities that readily absorb the acoustic sound waves over a particular frequency range. 

We claim:
 1. A liner for reducing internal acoustic noise comprising: a sound penetrable member; a second member, said second member spaced from said first member and cooperating with said sound penetrable member to form a hollow there between; a plurality of acoustic resonators positioned between said sound penetrable member and said second member, each of said plurality of acoustic resonators having an opening and an acoustical resonance cavity therein for ingress of a sound wave whereby the acoustical resonance cavity generates a resonance sound wave causing vibration displacement of each of said plurality of acoustic resonators thereby causing dissipation of the sound wave by conversion of acoustical energy into vibration energy.
 2. The liner of claim 1 wherein the plurality of acoustic resonators includes acoustic resonators of different mass.
 3. The liner of claim 1 wherein the plurality of acoustic resonators includes acoustic resonators of different length cavities.
 4. The liner of claim 3 wherein the plurality of acoustic resonators includes acoustic resonators of different mass.
 5. The liner of claim 1 wherein the first penetrable sound member comprises a mesh screen.
 6. The liner of claim 1 wherein the acoustic resonators comprises ceramic beads.
 7. The liner of claim 1 wherein the acoustic resonators comprises glass beads.
 8. The liner of claim 1 wherein the liner comprises a portion of a cabinet.
 9. The liner of claim 1 wherein the liner comprises an integral portion of a cabinet.
 10. The method of attenuation the noise within a cabinet comprising: placing a sound penetrable member within the cabinet; placing a second member in a spaced condition from the sound penetrable member to create a space there between; and placing a plurality of acoustic resonators in the space so that noise generated within the cabinet that impinges on the acoustic resonators is attenuated by translation of acoustical energy into vibration energy that displaces each of the plurality of acoustic resonators.
 11. The method of claim 10 wherein the step of placing a plurality of acoustic resonators in the space comprises placing acoustic resonators having cavities of different lengths therein.
 12. The method of claim 11 wherein the step of placing a plurality of acoustic resonators in the space comprises placing acoustic resonators of different external sizes in the space.
 13. The method of claim 12 wherein the step of placing a plurality of acoustic resonators in the space comprises placing acoustic resonators of different materials in the space.
 14. The method of claim 13 wherein the step of placing a plurality of acoustic resonators in the space comprises placing acoustic resonators of different mass in the space.
 15. The method of claim 14 wherein the step of placing a plurality of acoustic resonators in the space comprises placing a sound penetrating mesh on a side of the liner exposed to acoustical wave energy.
 16. A liner for reducing internal acoustic noise comprising: a sound penetrable member; a second member, said second member spaced from said first member and cooperating with said sound penetrable member to form a hollow there between; a plurality of acoustic resonators positioned between said sound penetrable member and said second member, each of said plurality of acoustic resonators spaced from an adjacent acoustic resonator therein to provide a labyrinth pathway for passage of a sound wave from said sound penetrable member to said second member thereby causing attenuation of the sound wave as the sound wave propagates through the labyrinth pathway. 